[3.14] gh-137335: Fix unlikely name conflicts for named pipes in multiprocessing...

[3.14] gh-137335: Fix unlikely name conflicts for named pipes in multiprocessing and asyncio on Windows (GH-137389) (GH-145170)

Since os.stat() raises an OSError for existing named pipe "\\.\pipe\...",
os.path.exists() always returns False for it, and tempfile.mktemp() can
return a name that matches an existing named pipe.

So, tempfile.mktemp() cannot be used to generate unique names for named
pipes. Instead, CreateNamedPipe() should be called in a loop with
different names until it completes successfully.
(cherry picked from commit d6a71f4690c702892644b1fbae90ae9ef733a8ab)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>